stroboscopic (aka equivalent time sampling) using EZCAP DVB dongle ?

friedtj friedtj at free.fr
Fri Jul 6 07:52:40 UTC 2012


I am trying to implement equivalent time sampling using an EZCAP dongle configured using
gnuradio-companion. Since I am not completely clear about the tasks of the E4000 wrt RTL2832,
it could be that I am missing a significant information, so here is my experimental setup aimed
at developing a monostatic pulse mode RADAR :
* I am using a radiofrequency acoustic delay line to generate 4 echos delayed 1 to 2 microseconds
after an incoming excitation pulse is generated by a frequency synthesizer. The excitation pulse
is 125 ns long, and repetition rate is 4 microseconds. The carrier frequency at 860 MHz is chopped
by a fast duplexer, one side being connected to the frequency synthesizer and the other sizer to the
EZCAP dongle
* the EZCAP is configured with an LO at 860 MHz, 2 MS/s output, and I plot |I+jQ| after keeping
only 1 every 8 sample (2 MS/s=500 ns delay between samples, and 1 every 8 samples means a spacing
of 4 us, close to the emitted pulse repetition rate)
* equivalent time sampling is obtained by slightly tuning the emitted pule repetition rate off the
4 us: I have checked that I can indeed obtain time stretching by tuning the pulse repetition rate
+/- 50 ns away from the 4 us dela, with time stretching factors of up to 10^5 (ie the 4 us pulses
are recorded as 400 ms traces (thus including 10^5 points or an equivalent sampling rate of 25 GS/s).

Now of course a stretching factor of 10^5 is overkill and the RF front end definitely does not have such
a huge bandwidth, this is just to demonstrate the concept. With a more reasonable stretching factor of 10 (ie
sampling every 4.4 us with a 4 us emission pulse repetition rate), I can expect to convert a 2 MS/s sampling rate 
to an equivalent time sampling of 20 MS/s, which would already improve my monostatic RADAR resolution by a factor 10,
and more or less fit my targetted resolution.

So now the questions:
* the 125 ns pulse I generate would span 8 MHz. From my reading of the E4k description, this is within the
IF bandwidth available. However, although I can observe the emitted pulse (0 dBm), I cannot see the echos
(-35 to -40 dBm), whatever LNA gain I use (from 0 to 40 dB). Is the default configuration of the E4k an IF of
8 MHz, or lower. What parameter should I give the rtlsdr gnuradio source block to make sure I have the right bandwidth ?
* since I am not clear about task distribution between the E4k and the RTL2832, is there another limitation that
will prevent a 8 MHz wide (bandwidth) signal to be recorded through the 2 MS/s I/Q output recorded ?

Thank you, Jean-Michel

-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France




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